Android LayoutInflater.inflate源码分析

layoutinflater.inflate源码详解

layoutinflater的inflate方法相信大家都不陌生,在fragment的oncreateview中或者在baseadapter的getview方法中我们都会经常用这个方法来实例化出我们需要的view.

假设我们有一个需要实例化的布局文件menu_item.xml:

<linearlayout xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:orientation="vertical">

  <textview
    android:id="@+id/id_menu_title_tv"
    android:layout_width="match_parent"
    android:layout_height="300dp"
    android:gravity="center_vertical"
    android:textcolor="@android:color/black"
    android:textsize="16sp"
    android:text="@string/menu_item"/>
</linearlayout>

我们想在baseadapter的getview()方法中对其进行实例化,其实例化的方法有三种,分别是:

2个参数的方法:

convertview = minflater.inflate(r.layout.menu_item, null);

3个参数的方法(attachtoroot=false):

convertview = minflater.inflate(r.layout.menu_item, parent, false);

3个参数的方法(attachtoroot=true):

convertview = minflater.inflate(r.layout.menu_item, parent, true);

究竟我们应该用哪个方法进行实例化view,这3个方法又有什么区别呢？如果有同学对三个方法的区别还不是特别清楚,那么就和我一起从源码的角度来分析一下这个问题吧.

源码

inflate

我们先来看一下两个参数的inflate方法,源码如下:

public view inflate(@layoutres int resource, @nullable viewgroup root) {
  return inflate(resource, root, root != null);
}

从代码我们看出,其实两个参数的inflate方法根据父布局parent是否为null作为第三个参数来调用三个参数的inflate方法,三个参数的inflate方法源码如下:

public view inflate(@layoutres int resource, @nullable viewgroup root, boolean attachtoroot) {
  // 获取当前应用的资源集合
  final resources res = getcontext().getresources();
  // 获取指定资源的xml解析器
  final xmlresourceparser parser = res.getlayout(resource);
  try {
    return inflate(parser, root, attachtoroot);
  } finally {
    // 返回view之前关闭parser资源
    parser.close();
  }
}

这里需要解释一下,我们传入的资源布局id是无法直接实例化的,需要借助xmlresourceparser.

而xmlresourceparser是借助android的pull解析方法是解析布局文件的.继续跟踪inflate方法源码:

public view inflate(xmlpullparser parser, @nullable viewgroup root, boolean attachtoroot) {
  synchronized (mconstructorargs) {
    // 获取上下文对象,即layoutinflater.from传入的context.
    final context inflatercontext = mcontext;
    // 根据parser构建xmlpullattributes.
    final attributeset attrs = xml.asattributeset(parser);
    // 保存之前的context对象.
    context lastcontext = (context) mconstructorargs[0];
    // 赋值为传入的context对象.
    mconstructorargs[0] = inflatercontext;
    // 注意,默认返回的是父布局root.
    view result = root;

    try {
      // 查找xml的开始标签.
      int type;
      while ((type = parser.next()) != xmlpullparser.start_tag &&
          type != xmlpullparser.end_document) {
        // empty
      }

      // 如果没有找到有效的开始标签,则抛出inflateexception异常.
      if (type != xmlpullparser.start_tag) {
        throw new inflateexception(parser.getpositiondescription()
            + ": no start tag found!");
      }

      // 获取控件名称.
      final string name = parser.getname();

      // 特殊处理merge标签
      if (tag_merge.equals(name)) {
        if (root == null || !attachtoroot) {
          throw new inflateexception("<merge /> can be used only with a valid "
              + "viewgroup root and attachtoroot=true");
        }

        rinflate(parser, root, inflatercontext, attrs, false);
      } else {
        // 实例化我们传入的资源布局的view
        final view temp = createviewfromtag(root, name, inflatercontext, attrs);
        viewgroup.layoutparams params = null;

        // 如果传入的parent不为空.
        if (root != null) {
          if (debug) {
            system.out.println("creating params from root: " +
                root);
          }
          // 创建父类型的layoutparams参数.
          params = root.generatelayoutparams(attrs);
          if (!attachtoroot) {
            // 如果实例化的view不需要添加到父布局上,则直接将根据父布局生成的params参数设置
            // 给它即可.
            temp.setlayoutparams(params);
          }
        }

        // 递归的创建当前布局的所有控件
        rinflatechildren(parser, temp, attrs, true);

        // 如果传入的父布局不为null,且attachtoroot为true,则将实例化的view加入到父布局root中
        if (root != null && attachtoroot) {
          root.addview(temp, params);
        }

        // 如果父布局为null或者attachtoroot为false,则将返回值设置成我们实例化的view
        if (root == null || !attachtoroot) {
          result = temp;
        }
      }

    } catch (xmlpullparserexception e) {
      inflateexception ex = new inflateexception(e.getmessage());
      ex.initcause(e);
      throw ex;
    } catch (exception e) {
      inflateexception ex = new inflateexception(
          parser.getpositiondescription()
              + ": " + e.getmessage());
      ex.initcause(e);
      throw ex;
    } finally {
      // don't retain static reference on context.
      mconstructorargs[0] = lastcontext;
      mconstructorargs[1] = null;
    }

    trace.traceend(trace.trace_tag_view);

    return result;
  }
}

上述代码中的关键部分我已经加入了中文注释.从上述代码中我们还可以发现,我们传入的布局文件是通过createviewfromtag来实例化每一个子节点的.

createviewfromtag

函数源码如下:

/**
 * 方便调用5个参数的方法,ignorethemeattr的值为false.
 */
private view createviewfromtag(view parent, string name, context context, attributeset attrs) {
  return createviewfromtag(parent, name, context, attrs, false);
}

view createviewfromtag(view parent, string name, context context, attributeset attrs,
    boolean ignorethemeattr) {
  if (name.equals("view")) {
    name = attrs.getattributevalue(null, "class");
  }

  // apply a theme wrapper, if allowed and one is specified.
  if (!ignorethemeattr) {
    final typedarray ta = context.obtainstyledattributes(attrs, attrs_theme);
    final int themeresid = ta.getresourceid(0, 0);
    if (themeresid != 0) {
      context = new contextthemewrapper(context, themeresid);
    }
    ta.recycle();
  }

  // 特殊处理“1995”这个标签(ps: 平时我们写xml布局文件时基本没有使用过).
  if (name.equals(tag_1995)) {
    // let's party like it's 1995!
    return new blinklayout(context, attrs);
  }

  try {
    view view;
    if (mfactory2 != null) {
      view = mfactory2.oncreateview(parent, name, context, attrs);
    } else if (mfactory != null) {
      view = mfactory.oncreateview(name, context, attrs);
    } else {
      view = null;
    }

    if (view == null && mprivatefactory != null) {
      view = mprivatefactory.oncreateview(parent, name, context, attrs);
    }

    if (view == null) {
      final object lastcontext = mconstructorargs[0];
      mconstructorargs[0] = context;
      try {
        if (-1 == name.indexof('.')) {
          view = oncreateview(parent, name, attrs);
        } else {
          view = createview(name, null, attrs);
        }
      } finally {
        mconstructorargs[0] = lastcontext;
      }
    }

    return view;
  } catch (inflateexception e) {
    throw e;

  } catch (classnotfoundexception e) {
    final inflateexception ie = new inflateexception(attrs.getpositiondescription()
        + ": error inflating class " + name);
    ie.initcause(e);
    throw ie;

  } catch (exception e) {
    final inflateexception ie = new inflateexception(attrs.getpositiondescription()
        + ": error inflating class " + name);
    ie.initcause(e);
    throw ie;
  }
}

在createviewfromtag方法中,最终是通过createview方法利用反射来实例化view控件的.

createview

public final view createview(string name, string prefix, attributeset attrs)
  throws classnotfoundexception, inflateexception {
  // 以view的name为key, 查询构造函数的缓存map中是否已经存在该view的构造函数.
  constructor<? extends view> constructor = sconstructormap.get(name);
  class<? extends view> clazz = null;

  try {
    // 构造函数在缓存中未命中
    if (constructor == null) {
      // 通过类名去加载控件的字节码
      clazz = mcontext.getclassloader().loadclass(prefix != null ? (prefix + name) : name).assubclass(view.class);
      // 如果有自定义的过滤器并且加载到字节码,则通过过滤器判断是否允许加载该view
      if (mfilter != null && clazz != null) {
        boolean allowed = mfilter.onloadclass(clazz);
        if (!allowed) {
          failnotallowed(name, prefix, attrs);
        }
      }
      // 得到构造函数
      constructor = clazz.getconstructor(mconstructorsignature);
      constructor.setaccessible(true);
      // 缓存构造函数
      sconstructormap.put(name, constructor);
    } else {
      if (mfilter != null) {
        // 过滤的map是否包含了此类名
        boolean allowedstate = mfiltermap.get(name);
        if (allowedstate == null) {
          // 重新加载类的字节码
          clazz = mcontext.getclassloader().loadclass(prefix != null ? (prefix + name) : name).assubclass(view.class);
          boolean allowed = clazz != null && mfilter.onloadclass(clazz);
          mfiltermap.put(name, allowed);
          if (!allowed) {
            failnotallowed(name, prefix, attrs);
          }
        } else if (allowedstate.equals(boolean.false)) {
          failnotallowed(name, prefix, attrs);
        }
      }
    }

    // 实例化类的参数数组(mconstructorargs[0]为context, [1]为view的属性)
    object[] args = mconstructorargs;
    args[1] = attrs;
    // 通过构造函数实例化view
    final view view = constructor.newinstance(args);
    if (view instanceof viewstub) {
      final viewstub viewstub = (viewstub) view;
      viewstub.setlayoutinflater(cloneincontext((context)args[0]))
    }
    return view;
  } catch (nosunchmethodexception e) {
    // ......
  } catch (classnotfoundexception e) {
    // ......
  } catch (exception e) {
    // ......
  } finally {
    // ......
  }
}

总结

通过学习了inflate函数源码,我们再回过头去看baseadapter的那三种方法,我们可以得出的结论是:

第一种方法使用不够规范, 且会导致实例化view的layoutparams属性失效.(ps: 即layout_width和layout_height等参数失效, 因为源码中这种情况的layoutparams为null).

第二种是最正确,也是最标准的写法.

第三种由于attachtoroot为true,所以返回的view其实是父布局listview,这显然不是我们想要实例化的view.因此,第三种写法是错误的.

感谢阅读，希望能帮助到大家，谢谢大家对本站的支持！